1. Field of Invention
The present invention relates to a method of fabricating a trench isolation structure using a reverse mask. More particularly, the present invention relates to a method of forming a reverse mask over a conformal oxide layer in order to fabricate a trench isolation structure. The method is capable of fabricating devices having a line width smaller than about 0.25 .mu.m and enhancing the degree of uniformity of chemical-mechanical polished (CMP) layer.
2. Description of Related Art
The purpose of using a conventional reverse mask is to reduce the dishing effect due to chemical-mechanical polishing (CMP) so that a uniformly polished layer is obtained.
FIGS. 1A through 1D are cross-sectional views showing the progression of manufacturing steps taken to fabricate a trench isolation structure using a reverse mask according to a conventional method. First, as shown in FIG. 1A, a semiconductor substrate 10 is provided. Next, a thin insulating layer 11 is formed over the substrate 10, and then a first trench 13, a second trench 15 and a third trench 17 are formed in the substrate 10. Width W2 of the second trench 15 and width W3 of the third trench 17 are both greater than width W1 of the first trench 13. Thereafter, a conformal insulating layer 12 is formed over the thin insulating layer 11, the first trench 13, the second trench 15 and the third trench 17. In FIG. 1A, recess cavities 14 and 16 can be seen above the conformal insulating layer 12 above the second trench 15 and the third trench 17, respectively.
Next, as shown in FIG. 1B, a reverse mask layer 18 is formed above the conformal insulating layer 12, and then the reverse mask layer 18 is patterned. This patterned reverse mask layer 18 covers all the trench structures including the first trench 13, the second trench 15 and the third trench 17.
Next, as shown in FIG. 1C, using the reverse mask layer 18 as a mask, the conformal insulating layer 12 is etched to form a patterned conformal insulating layer 12 underneath the reverse mask layer 18. Thereafter, the reverse mask layer 18 is removed forming a plurality of protruded insulating structures, for example, 12a, 12b and 12c.
Finally, as shown in FIG. 1D, a polishing operation, preferably a chemical-mechanical polishing, is carried out to polish the protruding insulating structures such as 12a, 12b and 12c until the semiconductor substrate 10 is exposed. Hence, trench isolation structures such as 13a, 15a and 17a are formed.
Although the aforementioned method of using a reverse mask to fabricate trench isolation structures is capable of removing defects due to the dishing effect, design rules for fabricating devices having line width smaller than 0.25 .mu.m are written having narrow limits. With little design tolerance, fabrication of a photomask must be controlled precisely, otherwise photo-misalignment can easily occur.
In light of the foregoing, there is a need to improve the method of using reverse masks to fabricate trench isolation structures.